Known in the present state of the art is a device for electrical discharge piercing of holes by means of a tool electrode (U.S. Pat. No. 4,543,460). The known device comprises an electrode holder which is in fact a guide element through whose bore an electrode is passed with a possibility to traverse vertically lengthwise its longitudinal axis under the action of an electrode feed drive. The drive is powered from a numerical control device whose compound signals (pulses) control the amount of spark gap between the electrode working end and the workpiece, as well as the rate of the electrode feed depthwise the workpiece. Besides, the electrode feed drive responds to electric signals delivered from the spark gap so as to adjust the position of the electrode working end with respect to the workpiece and to maintain constant spark gap. The amount of the spark gap is to be maintained constant also with the purpose of compensating for spark-erosion wear on the electrode. The electrode feed drive may perform also an additional function, i.e., to rotate the electrode about its longitudinal axis and impart oscillating motion to the electrode lengthwise its longitudinal axis.
Electrical discharge machining medium or working fluid is supplied under pressure from the tank along a pipe to the bore of the guide element and further on into the spark gap. In addition, the device is provided with a block which effects control over disposal of the spark-erosion waste products from the working gap so that concentration of said waste products be maintained somewhat above the preset level, thus preventing formation of electric discharge pulses from destabilization and making for stable rate of electrode feed and the entire workpiece machining process.
The block is situated in close proximity to the workpiece surface with a small clearance between the electrode and the workpiece; it has a centre hole for the electrode to pass through, a clearance being left between the hole faces and the electrode. An annular recess is provided in the top portion of said block, adapted to receive the bottom portion of the cylinder-shaped guide element carrying the electrode, both the former and the latter being held together. The centre hole in the block communicates with the bore of the guide element, wherein the electrode top portion is accommodated. The bottom block portion arranged lengthwise the workpiece surface, establishes a uniform clearance wtih the workpiece surface. The block is vertically traversable from its own drive, which is also controlled by command signals delivered from a numerical control device so as to cause the block to alter its vertical position with the purpose of maintaining the amount of clearance between the block lower surface and the workpiece surface constant. The drive is also capable of responding to a change in the amount of working (spark) gap and hence in the machining conditions.
However, the aforementioned electrode construction featuring the feed drive and compensation for the consumable electrode portion in conjunction with a working liquid recirculation in the machining zone renders the known device unsuitable for piercing deep or long holes whose diameter would be comparable with the electrode diameter, since the process of electrical discharge hole piercing according to the adopted technique will be accompanied by a continuously increasing consumption of electric power supplied to the electrode, while a major part of electric power will be spent for side spark erosion arising between the face of the hole being pierced and the bare electrode throughout its whole length equal to the length of hole being machined. Moreover, no working liquid recirculation in the machining zone leads to constant clogging of the spark gap with the products of the spark erosion process. Eventually, the hole piercing process ceases when the depth of the hole being machined gets equal to about 8 or 10 electrode diameters.